1. Field of the Invention
The present invention relates generally to luminaires of the type having a high pressure sodium lamp, a source of electrical power, and ballast-starter means normally connected to the power source and operative for supplying voltage of predetermined magnitude across the lamp to operate the latter. More particularly, the present invention relates to a cycling cut-off arrangement for and method of protecting the ballast-starter means from damage in the event of a cycling malfunction wherein the lamp is extinguished and the ballast-starter means applies a voltage of magnitude greater than said predetermined magnitude across the extinguished lamp.
2. Description of the Prior Art
Luminaires having a high pressure sodium lamp and a ballast-starter circuit normally connected to a source of electrical power and operative for supplying voltage to the lamp to operate the latter are well known, particularly for street lighting purposes. The operating characteristics of this sodium lamp are such that, as the lamp ages, some of the electrode material will deposit on the arc tube. This causes the arc tube to retain heat and, in turn, the internal pressure and the arc tube voltage will increase. When the arc tube voltage becomes so high that the ballast-starter circuit can no longer supply it, the lamp goes out. The lamp will restrike after it has cooled down sufficiently. This phenomenon of alternate lighting and extinguishing of the sodium lamp is commonly known as cycling.
Cycling is a major problem in the maintenance of street lighting. The ballast-starter circuit used in conventional luminaires will burn out if a cycling condition persists for thirty or more days. Unfortunately, it is very difficult to detect if a lamp is cycling in the field. By the time a serviceperson has arrived at the lamp location, the lamp may have come on again. The serviceperson will be very hesitant to remove an operating lamp, because replacement is expensive. Of course, if the cycling lamp is not replaced, the ballast-starter circuit will eventually also have to be replaced, thereby increasing the total maintenance cost of the system.
At present, a lamp replacement maintenance program serves to avoid the cycling problem. An average lamp working lifetime is determined, and the lamps are replaced before this working lifetime has expired. Since different lamps have different aging characteristics, this type of replacement program is a very expensive procedure for solving the cycling problem.